Focal adhesion kinase (FAK) regulates insulin-stimulated glycogen synthesis in hepatocytes

Abstract

Experimental data support a role for FAK, an important component of the integrin signaling pathway, in insulin action. To test the hypothesis that FAK plays a regulatory role in hepatic insulin action, we overexpressed wild type (WT), a kinase inactive (KR), or a COOH-terminal focal adhesion targeting (FAT) sequence-truncated mutant of FAK in HepG2 hepatoma cells. In control untransfected (NON) and vector (CMV2)- and WT-transfected cells, insulin stimulated an expected 54 ± 13, 37 ± 4, and 47 ± 12 increase in [U-14C]glucose incorporation into glycogen, respectively. This was entirely abolished in the presence of either KR (-1 ± 7%) or FAT mutants (0 ± 8%, n = 5, p < 0.05 for KR or FAT versus other groups), and this was associated with a significant attenuation of incremental insulin-stimulated glycogen synthase (GS) activity. Insulin-stimulated serine phosphorylation of Akt/protein kinase B was significantly impaired in mutant-transfected cells. Moreover, the ability of insulin to inactivate GS kinase-3β (GSK-3β), the regulatory enzyme immediately upstream of GS, by serine phosphorylation (308 ± 16, 321 ± 41, and 458 ± 34 optical densitometric units (odu) in NON, CMV2, and WT, respectively, p < 0.02 for WT versus CMV2) was attenuated in the presence of either FAT (205 ± 14, p < 0.01) or KR (189 ± 4, p < 0.005) mutants. FAK co-immunoprecipitated with GSK-3β, but only in cells overexpressing the KR (374 ± 254 odu) and FAT (555 ± 308) mutants was this association stimulated by insulin compared with NON (-209 ± 92), CMV2 (-47 ± 70), and WT (-39 ± 31 odu). This suggests that FAK and GSK-3β form both a constitutive association and a transient complex upon insulin stimulation, the dissociation of which requires normal function and localization of FAK. We conclude that FAK regulates the activity of Akt/protein kinase B and GSK-3β and the association of GSK-3β with FAK to influence insulin-stimulated glycogen synthesis in hepatocytes. Insulin action may be subject to regulation by the integrin signaling pathway, ensuring that these growth and differentiation-promoting pathways act in a coordinated and/or complementary manner.